1. Field of the Invention
The present invention relates to an optical scanning device that scans a scanning surface with a light beam emitted from a light source including a plurality of light-emitting units for an image forming apparatus and an image forming apparatus including the same.
2. Description of the Related Art
A tandem-type color image forming apparatus is widely used as a color image forming apparatus such as a color laser printer.
As a color-image forming apparatus such as a color laser printer, a tandem-type color-image forming apparatus is known. The tandem-type color-image forming apparatus typically includes a plurality of optical scanning units each including a plurality of scanning imaging lenses. Each of the optical scanning units scans a corresponding photosensitive element as a scanning surface, which is driven to rotate by a driving unit, with a laser beam to write image data of a corresponding color on the photosensitive element thereby forming an electrostatic latent image. The electrostatic latent image is developed into a toner image of the corresponding color by a developing unit. Then, toner images of different colors are transferred onto a printing sheet in a superimposed manner, thereby forming a full color image on the printing sheet.
Each of the optical scanning units emits laser beams with predetermined intervals therebetween from a semiconductor laser unit that is controlled to drive based on image signals of each color. The laser beams are focused on a uniformly charged surface of the photosensitive element via optical components such as a polygon mirror and a lens and scan the surface of the photosensitive element in a main-scanning direction. Furthermore, the laser beams scan the photosensitive element in a sub-scanning direction based on image signals corresponding to each color along with the rotation of the photosensitive element, to write the image data on the surface of the photosensitive element thereby forming the electrostatic latent images.
In an optical scanning device used in such a tandem-type color-image forming apparatus, structural components are arranged such that each of the laser beams that heads towards a corresponding photosensitive element passes through a different path. Therefore, a sub-scanning position of each of the laser beams that performs scanning is likely to differ for each color (fluctuation in shape of scan lines, so-called bend of a scan line, inclination of a scan line, and a sub-scanning write position (sub-scanning registration position)), thereby causing a color shift.
Specifically, when using a relatively inexpensive plastic scanning imaging lens, a shape of the plastic scanning imaging lens can be changed due to a change in temperature conditions during resin molding or an individual difference in molds at the time of multicavity molding, and the color shift becomes more severe. Moreover, the scanning imaging lens undergoes thermal deformation due to an impact of environmental temperature at a place where the color-image forming apparatus is installed or temperature rise within the color-image forming apparatus. Thus, a position of the optical beam is likely to change.
If a timing to write the electrostatic latent images on the photosensitive elements is not accurately adjusted among respective colors, a registration position shift (sub-scanning registration shift) occurs, thereby causing the color shift.
In existing technologies, such a scanning position shift is periodically detected at the time of apparatus startup or between jobs, etc. by using a registration-position-shift detection pattern that is registered on a transfer body and correction is carried out based on a result of detecting the pattern. However, at the time of continuous print operation, the scanning position further changes due to heat generation of a fixing vessel or a motor that drives the polygon mirror. Thus, if a large number of prints are necessary in a job, the color shift gradually increases.
In a technology disclosed in Japanese Patent Application Laid-open No. 2007-298563, for energy saving of a light source system, the color-image forming apparatus includes a splitting unit that splits a light beam FO emitted from the light source into at least a light beam FM and a light beam FS, and a detection optical system that optically guides the split light beam FM to a detecting unit. In a vertical section in a direction of the light beam FO, an optical component proximal to a point that is an intensity peak of the light beam FO is included only in the light beam FS without being included in the light beam FM. Thus, the light beam becomes substantially circular or elliptical and the intensity peak is formed at the substantial center.
The peak portion of the light is supplied to outside and the remaining portion of the light is detected for controlling an optical intensity. Thus, the optical intensity can be controlled and light can be efficiently supplied to outside. Consequently, increase in light use efficiency, energy saving, and reduction in environmental load can be realized.
In an existing technology, to achieve high speed and high density in the optical scanning device, a multibeam writing using a surface-emitting-laser light source is disclosed. However, with this optical scanning device, because the optical intensity is relatively low and the intensity fluctuation is caused by an environmental change, some problems arise such as that an optical-intensity-correction control needs to be performed with high accuracy (for example, instead of a back monitor photodiode (PD) such as the one used in an edge-emitting laser unit, a front monitor PD is required).
As described in the technology mentioned earlier, in the color-image forming apparatus that splits the light beam and detects the optical intensity, detection efficiency is degraded. Specifically, in the surface-emitting laser unit in which sufficient optical intensity is hardly obtained from the light source, accuracy in optical-intensity detection is degraded.